1. Field of the Invention
The present invention is related generally to preparation of thin-film compounds and more particularly to preparing thin-film compounds of Cu(In,Ga)(Se,S)2 in semiconductor devices.
2. Description of the Prior Art
Semiconductor alloy films of group IB-IIIA-VIA elements are the subject of considerable interest in the semiconductor field, particularly as to their utility as absorber layers in photovoltaic devices, such as solar cells. Photovoltaic devices (solar cells) utilize the specific electronic properties of semiconductors to convert the visible and near visible light energy of the sun into usable electrical energy. This conversion results from the absorption of radiant energy in the semiconductor materials, which frees some valence electrons, thereby generating electron-hole pairs. The energy required to generate electron-hole pairs in a semiconductor material is referred to as the band gap energy, which in general is the minimum energy needed to excite an electron from the valence band to the conduction band.
Semiconductor materials comprised of group IB and group IIIA metals and group VIA elements (commonly referred to as metal chalcogenides, or group IB-IIIA-VIA semiconductor thin-films) are important candidate materials for photovoltaic applications, since many of these semiconductor materials have optical band gap values well within the terrestrial solar spectrum. Mixed-metal chalcogenide semiconductors, such as copper-indium-diselenide (CuInSe2), copper-gallium-diselenide (CuGaSe2), and copper-indium-gallium-diselenide (CuIn1xe2x88x92X GaX Se2), all of which are sometimes generically referred to as Cu(In,Ga)Se2, or CI(G)S, have become the subject of considerable interest and study for semiconductor devices in recent years because of their high solar energy to electrical energy conversion efficiencies. Sulphur can also be, and sometimes is, substituted for selenium, so the CI(G)S is sometimes also referred to even more generically as Cu(In,Ga)(Se,S)2 to comprise all of those possible combinations. The electrical energy conversion efficiencies of CI(G)S semiconductor materials have been shown to exceed nineteen percent (19%) in active areas and to approach nineteen percent (19%) in total areas, which is quite high for current state-of-the-art solar cell technologies. See Contreras, M. et al., Progress in Photovoltaics (1999) 7:311-316. It has been generally believed by persons skilled in this art that the best electronic device properties, thus the best conversion efficiencies, are obtained when the Cu(In,Ga)Se2 compound or alloy is slightly Cu-poor.
While the growth of single crystal CuInSe2 has been studied, such as in U.S. Pat. No. 4,652,332, issued to T. Ciszek, the use of polycrystalline thin-films is really more practical. Sputter depositing a ternary single phase CuInSe2 layer, including the ability to determine the properties of the thin-film, such as multilayered structures, by varying the sputter process parameters, is described in U.S. Pat. No. 4,818,357, issued to Case et al.
Currently, the two fabrication methods of choice for the production of CI(G)S compounds are: (I) direct compound formation, such as physical vapor deposition of the constituent elements, exemplified by the process disclosed in U.S. Pat. No. 5,141,564, issued to Chen et al., which is generally used as a research tool, and (2) two-stage processes via various precursor structures, for example, the selenization of Cu/In metal precursors by either H2Se gas or Se vapor. The selenization technology generally exemplified by the processes described in U.S. Pat. No. 4,798,660, issued to Ermer et al., U.S. Pat. No. 4,915,745, issued to Pollock et al., and U.S. Pat. No. 5,045,409, issued to Eberspacher et al., is currently favored for manufacturing processes.
A general disadvantage of more of the direct absorber formation processes is the fact that high substrate temperatures (i.e., 300 to 550xc2x0 C.) need to be maintained during film growth for times exceeding 25 minutes. Although such substrate temperatures are acceptable on a laboratory scale, they are less desirable for industrial scale productions due to the high thermal budget. Most of the indirect two-stage processes address this problem by employing deposition of elemental copper, gallium, and indium onto unheated substrates followed by thermal treatment in the presence of selenium. However, most of these two-stage processes suffer from several drawbacks, such as requiring the use of highly toxic H2Se, which also has the drawback of resulting in low Se utilization, or poor film adhesion to Mo-coated substrates due to large volume expansion upon selenization of the dense elemental layer structures.
U.S. Pat. No. 5,356,839, issued to Tuttle et al., U.S. Pat. No. 5,441,897, issued to Noufi et al., and U.S. Pat. No. 5,436,204, issued to Albin et al. describe methods for producing high quality Cu(In,Ga)(Se,S)2 thin-films using vapor-phase recrystallization techniques. The fabrication processes described in these patents, each of which is assigned to the assignee of the present application, provide improved performance and yield as well as more reproducible, consistent quality than prior methods. For example, U.S. Pat. No. 5,356,839 describes a process for fabricating Cuw(In,Gay)Sez films by initially forming a Cu-rich, phase separated, compound mixture comprising Cu(In,Ga):CuxSe on a substrate, then converting the excess CuxSe to Cu(In,Ga)Se2 by exposing it to an activity of In and/or Ga, either in vapor form or in solid (In,Gay)Sez form. The characteristic of the resulting Cuw(In,Gay)Sez was controlled by the temperature. Higher temperatures, such as 500xc2x0-600xc2x0 C., resulted in a nearly stoichiometric Cu(In,Ga)Se2, whereas lower temperatures, such as 300xc2x0-400xc2x0 C., resulted in a more Cu-poor compound, such as the Cuz(In,Ga)4Se7 phase. U.S. Pat. Nos. 5,441,897 and 5,436,204 describe further modifications of the recrystallization process.
While the above-described metal chalcogenide semiconductor films provide relatively high conversion efficiencies, commercial use of these films is problematic for several reasons. Films produced by these selenization processes usually suffer from macroscopic spatial nonuniformities that degrade performance and yield, and reproducible consistent quality from run to run is unpredictable and difficult to obtain. Also, some of the key constituents of mixed-metal chalcogenide materials, such as indium and gallium, are very expensive, and current processes are somewhat wasteful of these materials. Finally, those processes in which precursors are deposited on heated substrates and subsequently are further heated to produce the mixed-metal chalcogenide film require a high thermal budget. Therefore, working with Cu(In,Ga)(Se,S)2 material has still been difficult, particularly when scaling up for industrial applications.
A need therefore exists for a fabrication process that produces a better quality, mixed-metal chalcogenide film more consistently and more predictably than previously known processes, and with more efficient and cost effective utilization of materials.
Accordingly, it is a general object of this invention to provide a novel, two-stage process for the production of monophasic group IIB-IIIA-VIA alloy semiconductor films.
It is a more specific object of this invention to provide a two-stage process that produces a device-quality Cu(In,Ga)(Se,S)2 more consistently and more predictably than previously known processes.
It is also an object of this invention to provide a process capable of fabricating high-quality films of Cu(In,Ga)(Se,S)2 with more efficient and cost effective utilization of materials.
It is another object of this invention to provide a method of producing Cu(In,Ga)(Se,S)2 precursors having approximately the same stoichiometry as the Cu(In,Ga)(Se,S)2 alloy produced by such precursors.
It is still another object of this invention to provide a process for producing Cu(In,Ga)(Se,S)2 thin-films wherein the process does not require an excess of any of the constituents of the thin-film or extensive heating times during processing, and thus can be scaled up easily to production of large areas and commercial quantities.
Additional objects, advantages, and novel features of the present invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following description and drawings, or may be learned by the practice of the invention, or may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention as embodied and broadly described herein, the two-stage method of this invention may comprise a first stage of depositing an amorphous group IB-IIIA-VIA precursor onto an unheated substrate, wherein the precursor comprises all of the group IB and group IIIA constituents of the semiconductor thin-film to be produced in the stoichiometric amounts desired for the final product, and subjecting the precursor to a short thermal treatment under an inert atmosphere to produce a single-phase, group IB-IIIA-VIA film. Preferably the precursor also comprises the group VIA element in the stoichiometric amount desired for the final semiconductor thin-film. The group IB-IIIA-VIA semiconductor films may be, for example, Cu(In,Ga)(Se,S)2 mixed-metal chalcogenides. The resultant substrate-supported group IB-IIIA-VIA semiconductor film is suitable for use in photovoltaic applications.